Photonic hydrogel sensors

Holographic surface relief diffraction gratings made of hydrogels for direct label-free biosensing of IgGs

This work describes the development of a label-free (LF) biosensing platform for the direct detection of targets based on diffractive structures fabricated with acrylamide-based hydrogels biofunctionalized with proteins and antibodies. Hydrogels containing Bovine Serum Albumin protein (BSA) with different crosslinking degrees were synthesized and characterized to find the optimal conditions for the suitable fabrication of surface relief gratings (SRGs). The bioavailability of BSA-functionalized hydrogels for the specific recognition of anti-BSA antibodies was verified by fluorescence detection. After the hydrogel-based SRG fabrication, diffraction efficiency measures at two different laser wavelengths were used for the direct LF detection of anti-BSA antibodies. The limit of detection in the sub mg L-1 range was read. Additionally, SRGs were prepared with hydrogels biofunctionalized with anti-rabbit antibodies for the direct detection of IgGs from rabbit serum, obtaining similar analytical performance without the necessity of labeling or applying amplification strategies.

3D-Bioprinted Hepar-on-a-Chip Implanted in Graphene-Based Plasmonic Sensors

Precise three-dimensional (3D) bioprinting designs enable the fabrication of unique structures for 3D-cell culture models. There is still an absence of real-time detection tools to effectively track in situ 3D-cell performance, hindering a comprehensive understanding of disease progression and drug efficacy assessment. While numerous bioinks have been developed, few are equipped with internal sensors capable of accurate detection. This study addresses these challenges by constructing a 3D-bioprinted hepar-on-a-chip embedded with graphene quantum dot-capped gold nanoparticle-based plasmonic sensors, featuring strong surface-enhanced Raman scattering (SERS) enhancement, biostability, and signal consistency. Such an integrated hepar-on-a-chip demonstrates excellent capability in the secretion of liver function-related proteins and the expression of drug metabolism and transport-related genes. Furthermore, the on-site detection of cell-secreted biomarker glutathione transferase α (GST-α) was successfully achieved using the plasmonic probe, with a dynamic linear detection range of 20-500 ng/mL, showcasing high anti-interference and specificity for GST-α. Ultimately, this integrated hepar-on-a-chip system offers a high-quality platform for monitoring liver injury.

Applications of inverse opal photonic crystal hydrogels in the preparation of acid-base color-changing materials

Hydrogels are three-dimensional (3D) crosslinked network hydrophilic polymers that have structures similar to that of biological protein tissue and can quickly absorb a large amount of water. Opal photonic crystals (OPCs) are a kind of photonic band gap material formed by the periodic arrangement of 3D media, and inverse opal photonic crystals (IOPCs) are their inverse structure. Inverse opal photonic crystal hydrogels (IOPCHs) can produce corresponding visual color responses to a change in acid or alkali in an external humid environment, which has wide applications in chemical sensing, anti-counterfeiting, medical detection, intelligent display, and other fields, and the field has developed rapidly in recent years. In this paper, the research progress on fast acid-base response IOPCHs (pH-IOPCHs) is comprehensively described from the perspective of material synthesis. The technical bottleneck of enhancing the performance of acid-base-responsive IOPCHs and the current practical application limitations are summarized, and the development prospects of acid-base-responsive IOPCHs are described. These comprehensive analyses are expected to provide new ideas for solving problems in the preparation and application of pH-IOPCHs.

Polysaccharide-based hydrogels for medical devices, implants and tissue engineering: A review

Hydrogels are highly biocompatible biomaterials composed of crosslinked three-dimensional networks of hydrophilic polymers. Owing to their natural origin, polysaccharide-based hydrogels (PBHs) possess low toxicity, high biocompatibility and demonstrate in vivo biodegradability, making them great candidates for use in various biomedical devices, implants, and tissue engineering. In addition, many polysaccharides also show additional biological activities such as antimicrobial, anticoagulant, antioxidant, immunomodulatory, hemostatic, and anti-inflammatory, which can provide additional therapeutic benefits. The porous nature of PBHs allows for the immobilization of antibodies, aptamers, enzymes and other molecules on their surface, or within their matrix, potentiating their use in biosensor devices. Specific polysaccharides can be used to produce transparent hydrogels, which have been used widely to fabricate ocular implants. The ability of PBHs to encapsulate drugs and other actives has been utilized for making neural implants and coatings for cardiovascular devices (stents, pacemakers and venous catheters) and urinary catheters. Their high water-absorption capacity has been exploited to make superabsorbent diapers and sanitary napkins. The barrier property and mechanical strength of PBHs has been used to develop gels and films as anti-adhesive formulations for the prevention of post-operative adhesion. Finally, by virtue of their ability to mimic various body tissues, they have been explored as scaffolds and bio-inks for tissue engineering of a wide variety of organs. These applications have been described in detail, in this review.

Hydrogels for active photonics

Conventional photonic devices exhibit static optical properties that are design-dependent, including the material's refractive index and geometrical parameters. However, they still possess attractive optical responses for applications and are already exploited in devices across various fields. Hydrogel photonics has emerged as a promising solution in the field of active photonics by providing primarily deformable geometric parameters in response to external stimuli. Over the past few years, various studies have been undertaken to attain stimuli-responsive photonic devices with tunable optical properties. Herein, we focus on the recent advancements in hydrogel-based photonics and micro/nanofabrication techniques for hydrogels. In particular, fabrication techniques for hydrogel photonic devices are categorized into film growth, photolithography (PL), electron-beam lithography (EBL), and nanoimprint lithography (NIL). Furthermore, we provide insights into future directions and prospects for deformable hydrogel photonics, along with their potential practical applications.

Advances in the Preparation of Tough Conductive Hydrogels for Flexible Sensors

The rapid development of tough conductive hydrogels has led to considerable progress in the fields of tissue engineering, soft robots, flexible electronics, etc. Compared to other kinds of traditional sensing materials, tough conductive hydrogels have advantages in flexibility, stretchability and biocompatibility due to their biological structures. Numerous hydrogel flexible sensors have been developed based on specific demands for practical applications. This review focuses on tough conductive hydrogels for flexible sensors. Representative tactics to construct tough hydrogels and strategies to fulfill conductivity, which are of significance to fabricating tough conductive hydrogels, are briefly reviewed. Then, diverse tough conductive hydrogels are presented and discussed. Additionally, recent advancements in flexible sensors assembled with different tough conductive hydrogels as well as various designed structures and their sensing performances are demonstrated in detail. Applications, including the wearable skins, bionic muscles and robotic systems of these hydrogel-based flexible sensors with resistive and capacitive modes are discussed. Some perspectives on tough conductive hydrogels for flexible sensors are also stated at the end. This review will provide a comprehensive understanding of tough conductive hydrogels and will offer clues to researchers who have interests in pursuing flexible sensors.

Application of colloidal photonic crystals in study of organoids

As alternative disease models, other than 2D cell lines and patient-derived xenografts, organoids have preferable in vivo physiological relevance. However, both endogenous and exogenous limitations impede the development and clinical translation of these organoids. Fortunately, colloidal photonic crystals (PCs), which benefit from favorable biocompatibility, brilliant optical manipulation, and facile chemical decoration, have been applied to the engineering of organoids and have achieved the desirable recapitulation of the ECM niche, well-defined geometrical onsets for initial culture, in situ multiphysiological parameter monitoring, single-cell biomechanical sensing, and high-throughput drug screening with versatile functional readouts. Herein, we review the latest progress in engineering organoids fabricated from colloidal PCs and provide inputs for future research.

Environmentally responsive hydrogel composites for dynamic body thermoregulation

Hydrogel composites exhibiting dynamic thermo-hydro responsive modulation of infrared radiation (IR) in the 5-15 μm range are designed for personalized body thermoregulation. Fabrication of the proposed system relies on the periodic arrangement of submicron-sized spherical fine silica (SiO₂) particles within poly(N-isopropylacrylamide) (PNIPAM)-based hydrogels. The dependence of the SiO₂ particles content on the IR reflection, followed by its modulation in response to any immediate environmental changes are thereby investigated. The addition of 20 wt% of SiO₂ allowed the hydrogel composites to reflect 20% of the IR emitted by the human body at constant temperature (i.e. T = 20 °C) and relative humidity (i.e. RH = 0%). According to Bragg's law, we found that the smaller the distance between the SiO₂ particles, the higher the IR reflection. The IR reflection further increased to a maximum of 42% when the resulting hydrogel composites are subjected to changes in relative humidity (i.e. RH = 60%) and temperature (i.e. T = 35 °C). Thermography is used to map the IR radiation emitted from the hydrogel composites when placed on the skin of the human body, demonstrating that the composite is actually reflecting IR. The latter results are supported by theoretical models that define the IR reflection profile of the resulting hydrogel composites with respect to the silica content, relative humidity and temperature.

Porous Hydrogels: Present Challenges and Future Opportunities

In this feature article, we critically review the physical properties of porous hydrogels and their production methods. Our main focus is nondense hydrogels that have physical pores besides the space available between adjacent cross-links in the polymer network. After reviewing theories on the kinetics of swelling, equilibrium swelling, the structure-stiffness relationship, and solute diffusion in dense hydrogels, we propose future directions to develop models for porous hydrogels. The aim is to show how porous hydrogels can be designed and produced for studies leading to the modeling of physical properties. Additionally, different methods that are used for making hydrogels with physically incorporated pores are briefly reviewed while discussing the potentials, challenges, and future directions for each method. Among kinetic methods, we discuss bubble generation approaches including reactions, gas injection, phase separation, electrospinning, and freeze-drying. Templating approaches discussed are solid-phase, self-assembled amphiphiles, emulsion, and foam methods.

Robust and Reliable Fabrication of Gelatin Films Containing Micropatterned Opal Structures via Evaporative Deposition and Thermal Gelation

Biopolymeric hydrogel materials containing tunable optical properties such as micropatterned artificial opal structures hold significant potential in various applications. Despite recent advances in fabrication techniques, simple, reliable, and tunable production of stimuli-responsive micropatterned opal hydrogels under mild conditions remains challenging. We report a simple micromolding-based evaporative deposition-thermal gelation technique for gelatin films that capture uniform opal micropatterns, aided by a potent aminopolysaccharide chitosan (CS) that provides binding affinity and structural stability. Our results show reliable, tunable, and high-fidelity fabrication of gelatin hydrogel films containing CS-opal micropatterns, while the as-prepared films show responsiveness to pH, ionic strength, and water content indicating a robust nature. Uniform CS-opal microparticles can also be readily prepared via removal of the gelatin through various simple routes, illustrating the crucial roles of CS and gelatin. We envision that this robust, reliable, and simple evaporative deposition-thermal gelation technique can be readily extended to prepare responsive biopolymeric materials for various applications.

Microfluidics-Assisted Fabrication of Dual Stopband Photonic Microcapsules and Their Applications for Anticounterfeiting

The assembly of two different kinds of colloidal particle-based photonic structures into an individual micro-object can achieve multifunctionality. In this study, core–shell photonic microcapsules with dual structural colors and photonic stop bands were prepared through a standard microfluidic technique. Photocurable resin suspension of silica nanoparticles and an aqueous suspension of nanogels were used as shell and core parts of microcapsules, respectively. The structural colors of shells and cores can be tuned by adjusting the concentrations of silica nanoparticles and soft nanogels in their corresponding suspensions. The individual microcapsules possess two distinct stop bands when the two suspensions are combined appropriately. Remarkably, the color information of the core part cannot be directly viewed at a macroscopic level (such as visual inspection) but can be detected at a microscopic scale (such as optical microscopy observation). The color information hidden enables the capability for information encryption and has potentially critical applications in anti-counterfeiting, display, and other fields.

Radioluminescent Photonic Bandgap Hydrogels: Mechanochromic Tunable Emissions

Fully organic, radioluminescent crystalline colloidal arrays (CCAs) with covalently incorporated emitters were synthesized by using up to three organic fluorophores that were Förster resonance energy transfer (FRET) pairs with each other. The emitters were covalently incorporated into monodisperse poly(styrene-co-propargyl acrylate) nanoparticles in various combinations, resulting in blue-, green-, and red-emitting CCAs when excited with an X-ray source. The negatively charged surfaces of the monodisperse nanoparticles caused self-assembly into a crystal-like structure, which resulted in a partial photonic bandgap (i.e., rejection wavelength) within the near-visible and visible light spectrum. When the rejection wavelength of the CCA overlapped its radioluminescence, the spontaneous emission was inhibited and the emission intensity decreased. A poly(ethylene glycol) methacrylate-based hydrogel network was used to encapsulate the CCAs and stabilize their crystal-like structure. Within the hydrogel, coupling the photonic bandgap with the radioluminescence of the CCA films led to robust optical systems with tunable emissions. These fully organic, hydrogel-stabilized, radioluminescent CCAs possess mechanochromic tunable optical characteristics with future applications as potentially less toxic X-ray bioimaging materials.

Regional Control of Multistimuli-Responsive Structural Color-Switching Surfaces by a Micropatterned DNA-Hydrogel Assembly

Structural colors have advantages compared with chemical pigments or dyes, such as iridescence, tunability, and unfading. Many studies have focused on developing the ability to switch ON/OFF the structural color; however, they often suffer from a simple and single stimulus, remaining structural colors, and target selectivity. Herein, we present regionally controlled multistimuli-responsive structural color switching surfaces. The key part is the utilization of a micropatterned DNA-hydrogel assembly on a single substrate. Each hydrogel network contains a unique type of stimuli-responsive DNA motifs as an additional cross-linker to exhibit swelling/deswelling via stimuli-responsive DNA interactions. The approach enables overcoming the existing limitations and selectively programming the DNA-hydrogel to a decrypted state (ON) and an encrypted state (OFF) in response to multiple stimuli. Furthermore, the transitions are reversible, providing cyclability. We envision the potential of our method for diverse applications, such as sensors or anticounterfeiting, requiring multistimuli-responsive structural color switching surfaces.

Femtosecond Laser Densification of Hydrogels to Generate Customized Volume Diffractive Gratings

Inspired by nature's ability to shape soft biological materials to exhibit a range of optical functionalities, we report femtosecond (fs) laser-induced densification as a new method to generate volume or subsurface diffractive gratings within ordinary hydrogel materials. We characterize the processing range in terms of fs laser power, speed, and penetration depths for achieving densification within poly(ethylene glycol) diacrylate (PEGDA) hydrogel and characterize the associated change in local refractive index (RI). The RI change facilitates the fabrication of custom volume gratings (parallel line, grid, square, and ring gratings) within PEGDA. To demonstrate this method's broad applicability, fs laser densification was used to generate line gratings within the phenylboronic acid (PBA) hydrogel, which is known to be responsive to changes in pH. In the future, this technique can be used to convert ordinary hydrogels into multicomponent biophotonic systems.

Repeatable detection of Ag+ ions using a DNA aptamer-linked hydrogel biochemical sensor integrated with microfluidic heating system

This paper describes repeatable detection of Ag⁺ ions using a DNA aptamer-linked hydrogel biochemical sensor integrated with a microfluidic heating system. Biochemical sensors that respond to chemical compounds and produce detectable signals have a critical role in many aspects of modern society. In particular, the repeatable measurement of environmental information such as toxic substances including Ag⁺ ions could be expected to improve the environment. The DNA aptamer is an attractive candidate because of the stability and the selectivity of binding to chemicals. However, previous DNA aptamer biochemical sensors could not measure repeatedly because those sensors did not have initializing functions. To overcome this challenge, we proposed a DNA aptamer-linked hydrogel biochemical sensor integrated with the microfluidic heating system enabling repeatable detection of Ag⁺ ions. The binding Ag⁺ ions are dissociated by heating and flushing through the integrated microfluidic heating device. The DNA aptamer-linked hydrogel had the capability to detect a wide range of Ag⁺ ion concentrations (10^-5-10 mM) including a toxic range for various aquatic organisms. Finally, we demonstrated the repeatable detection of the Ag⁺ ions. These results indicated that our proposed biochemical sensor is expected to use for long-term monitoring with high stability in ambient temperature and low power consumption.

Dual-Responsive Photonic Crystal Sensors Based on Physical Crossing-Linking SF-PNIPAM Dual-Crosslinked Hydrogel

Flexible wearable materials have frequently been used in drug delivery, healthcare monitoring, and wearable sensors for decades. As a novel type of artificially designed functional material, photonic crystals (PCs) are sensitive to the changes in the external environment and stimuli signals. However, the rigidity of the PCs limits their application in the field of biometric and optical sensors. This study selects silk fibroin (SF) and poly-N-isopropylacrylamide (PNIPAM) as principal components to prepare the hydrogel with the physical crosslinking agent lithium silicate (LMSH) and is then integrated with PCs to obtain the SF-PNIPAM dual-crosslinked nanocomposite for temperature and strain sensing. The structural colors of the PCs change from blue to orange-red by the variation in temperature or strain. The visual temperature-sensing and adhesion properties enable the SF-PNIPAM dual-crosslinked nanocomposite to be directly attached to the skin in order to monitor the real-time dynamic of human temperature. Based on its excellent optical properties and biocompatibility, the SF-PNIPAM dual-crosslinked nanocomposite can be applied to the field of visual biosensing, wearable display devices, and wound dressing materials.

Ramírez M, Zezza P, Lucío MI, Bañuls MJ, Maquieira Á, Morales-Vidal M, Beléndez A, Pascual I. Processing of Holographic Hydrogels in Liquid Media: A Study by High-Performance Liquid Chromatography and Diffraction Efficiency

The storage of time-stable holographic gratings in hydrogel matrices when the material is immersed in aqueous media is a real challenge at present. The optimization of the storage stages of the holograms must be properly investigated to identify the most suitable development processes. For this reason, this work is focused on the study of the optimization of the washing stages of the hydrogels based on acrylamide and N,N’-methylenebis(acrylamide) once unslanted transmission holograms have been stored. High-performance liquid chromatography and UV-visible measurements have been employed in our system to analyze the composition of the washing solutions. PBST and DMSO:H₂O are used as solvents in the washing stages. The diffraction efficiencies are measured during the washing stages and after the storing of the holograms during several days in PBST. Maximum diffraction efficiencies of 38 and 27.6% are reached when PBST and DMSO:H₂O are employed, respectively, for the washing process. Holograms show temporal stability after being stored immersed in PBST at 4 °C for 4 days.

Optical Indicators based on Structural Colored Polymers

Polymer indicators are autonomous responsive materials that provide an optical signal of a specific exposure in time. This review describes the different polymer systems utilized to obtain indicators based on structural color. Structural color originates from the interaction of light with a periodic nanostructured polymer which causes a specific wavelength to be reflected. This reflected light can be used for fabricating battery-free indicators that show visible structural color changes upon exposure to a stimulus or analyte. In this review, the typical structural color response types categorized by stimulus are discussed and compared. Furthermore, the steps toward possible applications of optical indicators based on structural colored polymers are outlined.

Development of a Photonic Switch via Electro-Capillarity-Induced Water Penetration Across a 10-nm Gap

With narrow and dense nanoarchitectures increasingly adopted to improve optical functionality, achieving the complete wetting of photonic devices is required when aiming at underwater molecule detection over the water-repellent optical materials. Despite continuous advances in photonic applications, real-time monitoring of nanoscale wetting transitions across nanostructures with 10-nm gaps, the distance at which photonic performance is maximized, remains a chronic hurdle when attempting to quantify the water influx and molecules therein. For this reason, the present study develops a photonic switch that transforms the wetting transition into perceivable color changes using a liquid-permeable Fabry-Perot resonator. Electro-capillary-induced Cassie-to-Wenzel transitions produce an optical memory effect in the photonic switch, as confirmed by surface-energy analysis, simulations, and an experimental demonstration. The results show that controlling the wetting behavior using the proposed photonic switch is a promising strategy for the integration of aqueous media with photonic hotspots in plasmonic nanostructures such as biochemical sensors.

Flory-Huggins VOC Photonics Sensor Made of Cellulose Derivatives

As a widespread air pollutant, volatile organic compounds (VOCs) are harmful to the human body's skin, nervous system, and respiratory system. Low-cost, extensive, and continuous detection of VOCs is of great significance to human health. We infiltrated and coated cellulose acetate on the inverse opal photonic crystal skeleton of methylcellulose-polyvinyl alcohol-graphene oxide to construct a degradable, high-toughness cellulose VOC sensor. Cellulose acetate enhances the response to VOCs and achieves a highly selective response to acetone vapor due to the smaller Flory-Huggins parameter with acetone. This work proposes a general, simple, easy-to-use, and highly selective photonic crystal VOC sensor development strategy. Calculated from the Flory-Huggins solution theory, a suitable polymer was selected to modify the inverse opal photonic crystal framework and achieve high selectivity detection.

Label-free detection of C-Reactive protein using bioresponsive hydrogel-based surface relief diffraction gratings

Responsive hydrogel-based surface relief gratings have demonstrated great performances as transducers in optical sensing. However, novel and smart designs of hydrogels are needed for the appropriate detection of analytes and biomolecules since the existing materials are very limited to specific molecules. In this work, a biosensing system based on surface relief gratings made of bioresponsive hydrogels has been developed. In particular, the hydrogel contains phosphocholine moieties to specifically recognize C-Reactive protein (CRP). The CRP-Sensing hydrogel capacity to selectively detect CRP was fully demonstrated. Using Direct Laser Interference Patterning, micro-gratings were created on CRP-Sensing hydrogel substrates and applied for the label-free sensing of CRP using a simple laser-based homemade optical setup. Limits of detection (LOD) and quantification (LOQ) in human serum dilutions of 1.07 and 8.92 mg L^-1, respectively, were reached. These results demonstrate that the biosensing system allows the selective label-free detection of CRP within concentration ranges around those related to risks of cardiovascular diseases and sepsis. Besides, amplification strategies have been carried out improving the sensitivity, widening the linear range, and reaching better LOD and LOQ (0.30 mg L^-1 and 4.36 mg L^-1). Finally, all the approaches were tested for the quantification of CRP in certified human serum with recoveries of around 100%.

High-Processable COF Gel Electrolyte Enabled by Side Chain Engineering for Lithium-ion Battery

Although covalent organic frameworks (COFs) with graphene-like structure present unique chemical and physical properties, they are essentially insoluble and infusible crystalline powders with poor processability, hindering their further practical applications. How to improve the processability of COF materials is a major challenge in this field. In this contribution, we proposed a general side chain engineering strategy to construct gel-state COF with high processability. This method takes advantages of large and soft branched alkyl side chains as internal plasticizers to achieve the gelation of COF, and systematically studied the influence of the length of the side chain on the COF gel formation. Benefit from their machinability and flexibility, this novel COF gel can be easily processed into gel-type electrolytes with specific sharpness and thickness, which were further applied to assemble the lithium ion batteries that exhibited high cycling stability.

Differences in the Electrochemical Performance of Pt-Based Catalysts Used for Polymer Electrolyte Membrane Fuel Cells in Liquid Half- and Full-Cells

A substantial amount of research effort has been directed toward the development of Pt-based catalysts with higher performance and durability than conventional polycrystalline Pt nanoparticles to achieve high-power and innovative energy conversion systems. Currently, attention has been paid toward expanding the electrochemically active surface area (ECSA) of catalysts and increase their intrinsic activity in the oxygen reduction reaction (ORR). However, despite innumerable efforts having been carried out to explore this possibility, most of these achievements have focused on the rotating disk electrode (RDE) in half-cells, and relatively few results have been adaptable to membrane electrode assemblies (MEAs) in full-cells, which is the actual operating condition of fuel cells. Thus, it is uncertain whether these advanced catalysts can be used as a substitute in practical fuel cell applications, and an improvement in the catalytic performance in real-life fuel cells is still necessary. Therefore, from a more practical and industrial point of view, the goal of this review is to compare the ORR catalyst performance and durability in half- and full-cells, providing a differentiated approach to the durability concerns in half- and full-cells, and share new perspectives for strategic designs used to induce additional performance in full-cell devices.

Water Resistant Cellulose Acetate Based Photopolymer for Recording of Volume Phase Holograms

The development of environmentally robust photosensitive materials for holographic recording is crucial for applications such as outdoor LED light redirection, holographic displays and holographic sensors. Despite the progress in holographic recording materials development, their sensitivity to humidity remains a challenge and protection from the environment is required. One approach to solving this challenge is to select substrate such as cellulose acetate, which is water resistant. This work reports the development of a cellulose-based photopolymer with sensitivity of 3.5 cm2/mJ and refractive index modulation of 2.5 × 10−3 achieved in the transmission mode of recording. The suitability for holographic recording was demonstrated by recording gratings with the spatial frequency of 800 linepairs/mm. The intensity dependence of the diffraction efficiency of gratings recorded in 70 μm thick layers was studied and it was observed that the optimum recording intensity was 10 mW/cm2. The robustness of the structures was studied after immersing the layer in water for one hour. It was observed that the diffraction efficiency and the surface characteristics measured before and after exposure to water remain unchanged. Finally, the surface hardness was characterized and was shown to be comparable to that of glass and significantly higher than the one of PVA-based acrylamide photopolymer.

Fabrication of inverse opal photonic gel sensors on flexible substrates by transfer process

We demonstrate a platform technology for transferring opal films and photonic gel films to flexible substrates. The conventional fabrication procedure for inverse opal photonic gel (IOPG) sensors comprises three major steps: 1) the self-assembly of polystyrene μ-spheres to an opal template film within a channel between the top and bottom substrates, 2) infiltration and photo-polymerisation of the monomer mixture, and 3) etching of the opal template. Owing to the low processing yield of the first step, it is difficult to fabricate multiple sensor arrays on a single substrate. In this study, an opal film is formed between two substrates with different surface polarities, and the film is separated by disassembling the two substrates. The opal film on a medium polar substrate is covered using a flexible polyethylene terephthalate (PET) film, and opal-templated photo-polymerisation is performed. Finally, the photonic gel with the opal template is transferred to the PET film, and the opal template is etched out. Using the platform technique, the fabrications of pH-responsive IOPG and temperature-responsive IOPG sensors on PET films are respectively demonstrated. In addition, the IOPG containing the copolymer of acrylamide and 3-acrylamidophenylboronic acid was found to be responsive to glucose at physiological pH. All three sensors were fabricated using the same transfer method, differing only in the composition of monomer mixtures, and they all showed excellent sensitivity and repeatability on PET substrates. Due to the advantageous feature of the transfer method, dual sensors of pH-responsive IOPG and temperature-responsive IOPG were sequentially fabricated on a single PET film.

Surface-Mounted Metal-Organic Frameworks: Past, Present, and Future Perspectives

Metal-organic frameworks (MOFs) are an emerging class of porous materials composed of organic linkers and metal centers/clusters. The integration of MOFs onto the solid surface as thin films/coatings has spurred great interest, thanks to leveraging control over their morphology (such as size- and shape-regulated crystals) and orientation, flexible processability, and easy recyclability. These aspects, in synergy, promise a wide range of applications, including but not limited to gas/liquid separations, chemical sensing, and electronics. Dozens of innovative methods have been developed to manipulate MOFs on various solid substrates for academic studies and potential industrial applications. Among the developed deposition methods, the liquid-phase epitaxial layer-by-layer (LPE-LbL) method has demonstrated its merits over precise control of the thickness, roughness, homogeneity, and orientations, among others. Herein, we discuss the major developments of surface-mounted MOFs (SURMOFs) in LbL process optimization, summarizing the SURMOFs' performance in different applications, and put forward our perspective on the future of SURMOFs in terms of advances in the formulation, applications, and challenges. Finally, future prospects and challenges with respect to SURMOFs growth will be discussed, keeping the focus on their widening applications.

Engineering Hydrogel-Based Biomedical Photonics: Design, Fabrication, and Applications

Light guiding and manipulation in photonics have become ubiquitous in events ranging from everyday communications to complex robotics and nanomedicine. The speed and sensitivity of light-matter interactions offer unprecedented advantages in biomedical optics, data transmission, photomedicine, and detection of multi-scale phenomena. Recently, hydrogels have emerged as a promising candidate for interfacing photonics and bioengineering by combining their light-guiding properties with live tissue compatibility in optical, chemical, physiological, and mechanical dimensions. Herein, the latest progress over hydrogel photonics and its applications in guidance and manipulation of light is reviewed. Physics of guiding light through hydrogels and living tissues, and existing technical challenges in translating these tools into biomedical settings are discussed. A comprehensive and thorough overview of materials, fabrication protocols, and design architectures used in hydrogel photonics is provided. Finally, recent examples of applying structures such as hydrogel optical fibers, living photonic constructs, and their use as light-driven hydrogel robots, photomedicine tools, and organ-on-a-chip models are described. By providing a critical and selective evaluation of the field's status, this work sets a foundation for the next generation of hydrogel photonic research.

Magnetically Assembled Photonic Crystal Gels with Wide Thermochromic Range and High Sensitivity

Thermochromic poly(N-isopropyl acrylamide) (PNIPAM) photonic crystal gels based on 1D magnetically assembling colloidal nanocrystal clusters have attracted much attention due to its convenient preparation process, striking color response, and good mechanical strength. However, there remain challenges to broaden the thermochromic range and improve the sensitivity for their iridescent color display. Here, a PNIPAM photonic gel with wide thermochromic range and high sensitivity is prepared by using four-arm star poly(ethylene glycol) acrylamide (PEGAAm) as cross-linker at appropriately reduced magnetic field strength as well as cross-linker content. PEGAAm improves the homogeneity of the microstructure in PNIPAM photonic gel and thus maintains the structure colors at a wide temperature range from room temperature to 44 °C. The reduced magnetic field strength of 70 Gs and low cross-linker content (the molar ratio of monomer to cross-linker of 300:1) lead to a large initial lattice spacing of the photonic gel and thus wide diffraction wavelength migration of 194 nm. This optimized PNIPAM gel exhibits vivid iridescent colors from orange-red to indigo blue as temperature increases from 20 to 44 °C with satisfactory repeatability. Therefore, it may be an ideal candidate for temperature sensors and displays with utility and accuracy such as low-temperature burns.

Zwitterionic Nanogels and Microgels: An Overview on Their Synthesis and Applications

Zwitterionic polymers by virtue of their unique chemical and physical attributes have attracted researchers in recent years. The simultaneous presence of positive and negative charges in the same repeat unit renders them of various interesting properties such as superhydrophilicity, which has significantly broadened their scope for being used in different applications. Among polyzwitterions of different architectures, micro- and/or nano-gels have started receiving attention only until recently. These 3D cross-linked colloidal structures show peculiar characteristics in context to their solution properties, which are attributable either to the comonomers present or the presence of different electrolytes and biological specimens. In this review, a concise yet detailed account is provided of the different synthetic techniques and application domains of zwitterion-based micro- and/or nanogels that have been explored in recent years. Here, the focus is kept solely on the "polybetaines," which have garnered maximum research interest and remain the extensively studied polyzwitterions in literature. While their vast application potential in the biomedical sector is being detailed here, some other areas of scope such as using them as microreactors for the synthesis of metal nanoparticles or making smart membranes for water-treatment are discussed in this minireview as well.

Doubly Photopolymerized Holographic Sensors

Holographic sensors are two-dimensional (2D) photonic crystals that diffract narrow-band light in the visible spectrum to quantify analytes in aqueous solutions. Here, a holographic fabrication setup was developed to produce holographic sensors through a doubly polymerization system of a poly-2-hydroxyethyl methacrylate hydrogel film using a pulsed Nd:YAG laser (λ = 355 nm, 5 ns, 100 mJ). Wavelength shifts of holographic Bragg peak in response to alcohol species (0-100 vol %) were characterized. Diffraction spectra showed that the holographic sensors could be used for short-chain alcohols at concentrations up to 60 vol %. The reversibility of the sensor was demonstrated, exhibiting a response time of 7.5 min for signal saturation. After 30 cycles, the Bragg peak and color remained the same in both 20 and 60 vol %. The fabrication parameters were simulated in MATLAB using a 2D finite-difference time-domain algorithm to model the interference pattern and energy flux profile of laser beam recording in the hydrogel medium. This work demonstrates a particle-free holographic sensor that offers continuous, reversible, and rapid colorimetric readouts for the real-time quantification of alcohols.

CLEAR - Contact lens technologies of the future

Contact lenses in the future will likely have functions other than correction of refractive error. Lenses designed to control the development of myopia are already commercially available. Contact lenses as drug delivery devices and powered through advancements in nanotechnology will open up further opportunities for unique uses of contact lenses. This review examines the use, or potential use, of contact lenses aside from their role to correct refractive error. Contact lenses can be used to detect systemic and ocular surface diseases, treat and manage various ocular conditions and as devices that can correct presbyopia, control the development of myopia or be used for augmented vision. There is also discussion of new developments in contact lens packaging and storage cases. The use of contact lenses as devices to detect systemic disease has mostly focussed on detecting changes to glucose levels in tears for monitoring diabetic control. Glucose can be detected using changes in colour, fluorescence or generation of electric signals by embedded sensors such as boronic acid, concanavalin A or glucose oxidase. Contact lenses that have gained regulatory approval can measure changes in intraocular pressure to monitor glaucoma by measuring small changes in corneal shape. Challenges include integrating sensors into contact lenses and detecting the signals generated. Various techniques are used to optimise uptake and release of the drugs to the ocular surface to treat diseases such as dry eye, glaucoma, infection and allergy. Contact lenses that either mechanically or electronically change their shape are being investigated for the management of presbyopia. Contact lenses that slow the development of myopia are based upon incorporating concentric rings of plus power, peripheral optical zone(s) with add power or non-monotonic variations in power. Various forms of these lenses have shown a reduction in myopia in clinical trials and are available in various markets.

Is the Microgel Collapse a Two-Step Process? Exploiting Cononsolvency to Probe the Collapse Dynamics of Poly-N-isopropylacrylamide (pNIPAM)

Many applications of responsive microgels rely on the fast adaptation of the polymer network. However, the underlying dynamics of the de-/swelling process of the gels have not been fully understood. In the present work, we focus on the collapse kinetics of poly-N-isopropylacrylamide (pNIPAM) microgels due to cononsolvency. Cononsolvency means that either of the pure solvents, e.g., pure water or pure methanol, act as a so-called good solvent, leading to a swollen state of the polymer network. However, in mixtures of water and methanol, the previously swollen network undergoes a drastic volume loss. To further elucidate the cononsolvency transition, pNIPAM microgels with diameters between 20 and 110 μm were synthesized by microfluidics. To follow the dynamics, pure water was suddenly exchanged with an unfavorable mixture of 20 mol% methanol (solvent-jump) within a microfluidic channel. The dynamic response of the microgels was investigated by optical and fluorescence microscopy and Raman microspectroscopy. The experimental data provide unique and detailed insight into the size-dependent kinetics of the volume phase transition due to cononsolvency. The change in the microgel's diameter over time points to a two-step process of the microgel collapse with a biexponential behavior. Furthermore, the dependence between the two time constants from this biexponential behavior and the microgel's diameter in the collapsed state deviates from the square-power law proposed by Tanaka and Fillmore [ J. Chem. Phys. 1979, 70, 1214-1218]. The deviation is discussed considering the adhesion-induced deformation of the gels and the physical processes underlying the collapse.

Multifunctional Composite Inverse Opal Film with Multiactives for Wound Healing

A film with an elaborate microstructure and multifunctions is urgently needed in wound healing. Here, we present a multiactive encapsulated inverse opal film with a monitorable delivery system for chronic wound healing. The inverse opal film is prepared by using poly(lactic-co-glycolic acid) to negatively replicate a colloidal crystal template, which presents a high specific surface area and interconnected nanopores. It could be imparted with a potent antibacterial effect and promote angiogenesis by loading the vascular endothelial growth factor into the nanopores and encapsulating by chitosan. In addition, it is demonstrated that the structure color change of the film could intuitively reflect the drug release progress from the nanopores, which made the film a real-time drug monitoring system. In the affected wound model, the properties of the multifunctional film in promoting wound healing are certified by the faster healing speed, more granulation tissue, less inflammation, and even a distribution of new blood vessels and collagen. These results indicate that the resultant multifunctional film has a practical application value in clinical wound care.

Preparations of Tough and Conductive PAMPS/PAA Double Network Hydrogels Containing Cellulose Nanofibers and Polypyrroles

To afford an intact double network (sample abbr.: DN) hydrogel, two-step crosslinking reactions of poly(2-acrylamido-2-methylpropanesulfonic acid) (i.e., PAMPS first network) and then poly(acrylic acid) (i.e., PAA second network) were conducted both in the presence of crosslinker (N,N'-methylenebisacrylamide (MBAA)). Similar to the two-step processes, different contents of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) oxidized cellulose nanofibers (TOCN: 1, 2, and 3 wt.%) were initially dispersed in the first network solutions and then crosslinked. The TOCN-containing PAMPS first networks subsequently soaked in AA and crosslinker and conducted the second network crosslinking reactions (TOCN was then abbreviated as T for DN samples). As the third step, various (T-)DN hydrogels were then treated with different concentrations of FeCl3(aq) solutions (5, 50, 100, and 200 mM). Through incorporations of ferric ions into (T-)DN hydrogels, notably, three purposes are targeted: (i) strengthen the (T-)DN hydrogels through ionic bonding, (ii) significantly render ionic conductivity of hydrogels, and (iii) serve as a catalyst for the forth step to proceed with in situ chemical oxidative polymerizations of pyrroles to afford polypyrrole-containing (sample abbr.: Py) hydrogels [i.e., (T-)Py-DN samples]. The characteristic functional groups of PAMPS, PAA, and Py were confirmed by FT-IR. Uniform microstructures were observed by cryo scanning electron microscopy (cryo-SEM). These results indicated that homogeneous composites of T-Py-DN hydrogels were obtained through the four-step process. All dry samples showed similar thermal degradation behaviors from the thermogravimetric analysis (TGA). The T2-Py5-DN sample (i.e., containing 2 wt.% TOCN with 5 mM FeCl3(aq) treatment) showed the best tensile strength and strain at breaking properties (i.e., σTb = 450 kPa and εTb = 106%). With the same compositions, a high conductivity of 3.34 × 10-3 S/cm was acquired. The tough T2-Py5-DN hydrogel displayed good conductive reversibility during several "stretching-and-releasing" cycles of 50-100-0%, demonstrating a promising candidate for bioelectronic or biomaterial applications.

Optical Sensing and Imaging of pH Values: Spectroscopies, Materials, and Applications

This is the first comprehensive review on methods and materials for use in optical sensing of pH values and on applications of such sensors. The Review starts with an introduction that contains subsections on the definition of the pH value, a brief look back on optical methods for sensing of pH, on the effects of ionic strength on pH values and pKa values, on the selectivity, sensitivity, precision, dynamic ranges, and temperature dependence of such sensors. Commonly used optical sensing schemes are covered in a next main chapter, with subsections on methods based on absorptiometry, reflectometry, luminescence, refractive index, surface plasmon resonance, photonic crystals, turbidity, mechanical displacement, interferometry, and solvatochromism. This is followed by sections on absorptiometric and luminescent molecular probes for use pH in sensors. Further large sections cover polymeric hosts and supports, and methods for immobilization of indicator dyes. Further and more specific sections summarize the state of the art in materials with dual functionality (indicator and host), nanomaterials, sensors based on upconversion and 2-photon absorption, multiparameter sensors, imaging, and sensors for extreme pH values. A chapter on the many sensing formats has subsections on planar, fiber optic, evanescent wave, refractive index, surface plasmon resonance and holography based sensor designs, and on distributed sensing. Another section summarizes selected applications in areas, such as medicine, biology, oceanography, bioprocess monitoring, corrosion studies, on the use of pH sensors as transducers in biosensors and chemical sensors, and their integration into flow-injection analyzers, microfluidic devices, and lab-on-a-chip systems. An extra section is devoted to current challenges, with subsections on challenges of general nature and those of specific nature. A concluding section gives an outlook on potential future trends and perspectives.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Structural colouration in the Himalayan monal, hydrophobicity and refractive index modulated sensing

The Himalayan monal is a bird in the pheasant family, and it is the national bird of Nepal. The bird possesses spectacular iridescent plumage with a range of different metallic colours. Here, we have studied the internal structure of its feathers from different parts of the bird's body and showed that its beautiful colours and iridescence are due to photonic structures present in the internal structure of the feathers. Sharp changes in the reflected brilliance were observed from the feathers upon changing the illumination conditions, such as horizontal and azimuthal angles. The feathers exhibited interesting hydrophobic properties, with the dull-coloured proximal end showing lower hydrophobicity with a contact angle between 90° and 110° compared with the iridescent distal end of a feather exhibiting a contact angle between 115° and 120°, attributed to the change in the internal structure and/or density of the feathers. A quick reversible change in colours of these feathers was observed when they were soaked in water and other liquids, which reversed upon drying. The shift in colour was suggested to be due to the swelling of the keratin layer of barbules that absorbed liquids and as a result modified the refractive index and periodicity of the internal photonic structures. The colour shift response of feathers was different in the case of alcohols and other water-based solutions, suggesting different swelling behaviour of keratin against different liquids; the water-based solution had the more pronounced effect. Such photonic modulation can be utilized in colour selective filters and sensing devices.

Osmosis manipulable morphology and photonic property of microcapsules with colloidal nano-in-micro structure

Full visible spectrum photonic droplets and consequent microcapsules with nano-in-micro structure were prepared through microfluidic technique. Photo-curable resin and suspension of monodispersed soft nanogels were used as shell and core of the microcapsules, respectively. Upon UV irradiation, the droplets can be subsequently transformed into photonic microcapsules with an ultrathin polymeric shell. The shell thickness of the photonic microcapsules was found to be approximately 700 nm. Due to the ultrathin shell and soft core, the photonic microcapsules with nano-in-micro structure display dramatic changes both in shapes and photonic property under the impact of osmosis effect or temperature stimulus. Typically, the shell and core parts of nano-in-micro structure could respectively undergo a size expansion/even rupture and a size decrease/buckling under hypotonic and hypertonic condition. Correspondingly, the peak value of the reflection spectra of the microcapsules showed a redshift and blue shift, respectively. The mechanism to the structure and optical properties variation involves the osmotic pressure induced the volume-fraction change of the nanogel-based photonic dispersion and the shell buckling of the core/shell microcapsules.